1. Do research. Using Wikipedia and Google, learn what a pH meter is, how a pH meter works, and what the major components are. Learn the unique requirements of the circuits involved.
2. Search the internet for pH meter circuits. Study the schematics until you understand their operation.
3. Decide which features you want your pH meter to have; digital display or analog meter, line power or battery power, accuracy, controls and adjustments, etc.
4. Create a block diagram showing all of the major system components and how they interact.
5. Draw schematics for the power, signal processing, and display systems.
6. Write.
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